A security system utilizing a cable TV communications system is disclosed in a pending patent application entitled "Security System", by Tom O'Brien, Ser. No. 328,304, filed Dec. 7, 1981, which application is assigned to the assignee of the present invention. The latter patent application describes an alarm system utilizing three radio frequency signals as data communication channels between a headend alarm processor and a plurality of subscriber alarm processors. One channel is a polling channel on which the headend alarm processor transmits command messages to all subscriber units. Another channel is a polling response channel on which an addressed subscriber unit responds to a headend command message. The third channel is an alarm channel on which subscriber units transmit alarm messages to the headend essentially when a respective alarm condition is first detected.
When two or more subscriber units attempt to transmit simultaneous alarm messages on the alarm channel, the resulting collision causes a garbled alarm message to be received at the headend. Other factors may also cause garbled alarm messages to be received at the headend. For example, a malfunctioning subscriber unit may transmit a message containing an error, e.g. a parity bit error, which will be interpreted at the head end as a garbled message. As another example, noise on the cable can garble an alarm message.
In a security system, garbled alarm messages cannot be ignored because the alarm message represents an emergency condition. Although it is possible to sequentially poll all subscriber units individually to discover which unit originated the alarm message, sequential polling tends to reduce the system response time to an alarm. Therefore, it is desirable that the security system have the capability to search for and verify the subscriber unit or units that have originated the garbled message or messages.